This invention relates in general to the growth of crystalline ribbons. More specifically, it relates to an improved guidance system for silicon ribbons being grown generally horizontally using a "low angle" process.
The most common approach to the growth of crystalline ribbons, particularly single crystal ribbons of silicon or related semiconductor materials, has been vertical growth from a melt using dies to shape the crystal. U.S. Pat. Nos. 4,248,645 and 4,267,100 are representative of this approach and the '100 patent describes a parallel plate guidance system compatible with this vertical growth technique.
A more recent approach is to grow the ribbon from a melt at a small angle above the horizontal. This "low angle silicon sheet" (LASS) technique is described in detail in U.S. Pat. No. 4,289,571 to one of the present applicants, which is incorporated herein by reference. In general, the LASS technique uses a shallow crucible that holds a melt of liquid silicon. A "scraper" is mounted in the crucible at one end of a growth zone. The melt temperature in the growth zone is carefully adjusted to a value that produces crystalline growth at a very thin leading edge of a seed crystal and then the growing crystal ribbon. A puller mechanism draws the seed crystal and the continuous, single crystal ribbon from the melt at the aforementioned small positive angle. Because of this angle, a meniscus of liquid silicon forms under the solid ribbon and above the surface of the melt. The scraper, which projects slightly above the melt surface, blocks the lower end of the meniscus. The upper end of the meniscus continually detaches from the underside of the ribbon during the pulling.
Several important aspects of this growth technique are that (1) the growth dynamics at the leading edge are sensitive to temperature variations and (2) it is extremely difficult to produce and maintain a stable temperature profile that produces solidification at the leading edge while also avoiding the growth of dendrites or the growth of a ribbon with dislocations or a high degree of non-uniformity in its cross-sectional dimensions.
With the LASS technique it is also necessary to support and guide the as grown ribbon, which is at a high temperature and still somewhat plastic, in the region between the scraper and the puller. Heretofore the principal solution to this problem was the straightforward expedient of placing a rigid, inclined guide plate or "ramp" under the ribbon adjacent to the scraper and co-planar with the nip of the rollers or belts of the puller. This arrangement, however, exhibits a serious problem. The ribbon can contain irregularities on its bottom surface, typically small (6 mm wide by 3 mm high or smaller) "platelets" of SiC. When these irregularities reach the ramp, because the ramp is fixed, the entire ribbon is raised by the height of the irregularlity. This in turn raises the end of the ribbon in the melt defined by the growth zone. This upward movement at the melt disturbs the orderly formation of the crystal at the leading edge. Orderly crystal growth is a very delicate phenomenon, critically dependent on proper maintenance of temperature gradients in and adjacent to the growth zone. Therefore, elevation of the leading edge by even one millimeter can disrupt the desired orderly growth. Such a disruption takes many forms including dendritic growth and thickening of the forming crystal ribbon. The upward movement at the leading edge creates a meniscus between the leading edge and the liquid melt. Crystal growth proceeds down the meniscus to create a wave in the bottom surface of the ribbon. When this disruption wave reaches the ramp, it also elevates the entire ribbon to repeat the process, but usually with an increased amplitude in the second wave. This problem not only produces a ribbon with varying dimensions, but more significantly it eventually disrupts the entire growth process.
Previous attempts to control this wave problem have, in general, focused on controlling the vertical movement of the ribbon close to the scraper. One solution was to support the ribbon on a ramp as before, but also to top load the ribbon with a set of rollers. This system had no material affect on the "wave" problem. Another approach was to place the ramp above the ribbon with its bottom supported by a set of rollers mounted in a frame with multiple independent pivot axes arrayed along the pulling direction. This arrangement readily accommodates variations in the bottom surface, but underside support structure was so flexible that it would twist, it would not support the ribbon well, and most significantly, the growth end of the ribbon would continue to move to an unacceptable degree in response to the bottom surface irregularities. Another solution was to create a variable gap near the crucible using upper and lower plates that sandwiched the ribbon. Pivoted link arms connected the plates and there was an arrangement for gradually moving the upper plate downwardly to narrow the gap and thereby control the vertical excursions of the ribbon. This approach also showed little or no improvement over the plain ramp. Some improvement was achieved using a long bottom ramp between the scraper and the puller, but with a graphite felt covering on its upper surface. This arrangement, however, is not ideal because the growth end of the ribbon is not fully stabilized and because the hot ribbon deteriorates the felt. Therefore, while some improvement is observed for short lengths of ribbon (e.g. 10 to 20 m), this guidance and support system is not satisfactory for continuous commercial production of ribbon in lengths of hundreds of meters. In summary, no known guidance system for the LASS growth technique has controlled the wave problem to a degree that is acceptable for use in commercial production.
It is therefore a principal object of this invention to provide a support and guidance system for crystalline ribbon, particularly single crystal silicon ribbons grown by the LASS technique, which substantially eliminates vertical movements of the ribbon in the growth zone due to irregularities on the bottom surface of the as grown ribbon.
Another object is to provide a system with the foregoing advantage that is uncomplicated, cost effective and fully compatible with known LASS equipment.
A further object is to provide a system with the foregoing advantages that is sufficiently reliable and durable for use in continuous commercial production of silicon ribbon.